Attempts have been made to create coaxial lines by coating a wire with a thin dielectric material on which a deposit of copper and/or lead-tin is vapor deposited with the coaxial segments therein inserted into a metallic body having a predrilled hole, the inside of which is also vapor deposited with a thin coating of lead-tin. Such coaxial line interposers are then formed by soldering the segments to the metallic fixture.
In the development of a very high speed tester for integrated circuits, it is desirable to maintain a uniform 50 ohm signal impedance from the driver/receiver electronics to the device under test (DUT). At the same time, to minimize power distribution noise generated by the rapid change of current through a power lead, a low impedance for the power path is required.
In the fabrication of such rigid probe interposers, there are three major steps required to achieve a coaxial design for both the signal and voltage probes constituted by the integrated structure. The first is the fabrication of the voltage probe. In a typical prior design, a thin (3,000 .ANG.) film of tantalum oxide is deposited on a 0.006 inch wire probe and a coating of copper and/or lead-tin is sputtered onto the outside surface of the oxide to form a low impedance coaxial segment. This involves several significant fabrication problems which must be overcome. First, the surface of the wire must be completely free of metal fragments and depressions on its surface so that the oxide can coat uniformly and so that no metal fragments are exposed. Secondly, when depositing the copper and/or lead-tin coating to the outside surface of the oxide, the possibility of pin holes or surface cracks in the oxide leads to an exposure of shorting the copper coating to the center conductor.
For the signal probe in such typical prior design, the fabrication entails depositing parylene on a 0.002 inch diameter wire to an overall thickness of 0.006 inches. Then chrome, copper and lead-tin is evaporated on the parylene surface. Again, a problem arises as there may be voids in the dielectric coatings such that the metallic deposit may short to the center conductor, rendering the probe useless.
The final process step is to form the completed interposer and integrating the voltage probe and the signal probe. The preformed coaxial segments for the voltage and signal probes are inserted into the required locations of a predrilled laminated brass body having parallel holes receiving the voltage and signal probes. The laminated sections have a deposit of lead-tin on each face. Once this is done, the entire fixture is brought above the melting temperature of the lead-tin to sweat all elements together. Then polyimide is applied to the probe surface for electrical isolation and stiffening of the exposed probe wire.
In a coaxial line formed of a central conductor covered with a thin layer of insulation, e.g. 3,000 .ANG. of Ta.sub.2 O.sub.5 and an outer metal sheath, e.g. deposited lead-tin or copper or both, voids in the dielectric material may provide a path for shorting the sheath to the central conductor.
It is therefore an object of the present invention to provide an improved rigid probe interposer for connecting a very high speed tester electronics to an integrated circuit under test in which the interposer is manufactured through a single step encapsulation of both signal and voltage probes, good electrical characteristics are provided for both probes, the interposer is of simplified construction and has good mechanical stability for extensive repetitive use.